Magnetic record testing means



APlll 28, 1959 D. K. REYNOLDS MAGNETIC RECORD TESTING MEANS OriginalFile'd Nov. 23, 1953 Re. 24,641 Reissued Apr. 28, 1959 Unitedl States gPatent Oice MAGNETIC RECORD TESTING MEANS Donald K. Reynolds, Seattle,Wash., assignor, by mesne assignments, to General Electric Company, NewYork, N.Y., a corporation of New York Original No. 2,793,344, dated May21, 1957, Serial No. 393,869, November 23, 1953. Application for reissueJanuary 16, 1958, Serial No. 709,429

9 Claims. (Cl. 324-34) Matter enclosed in heavy brackets appears in theoriginal patent but forms no part of this reissue specification; matterprinted in italics indicates the additions made by reissue.

nodules, as they are called, do appear, despite these precautions on thepart of manufacturers. Present day information handling machines must beaccurate, since `they are being employed now in accounting systems andinventory control systems, and, indeed, even in banking systems.Therefore, great lengths are gone to, to insure their accuracy. Errorchecking and error correcting codes are employed, and, in manyinstances, there is duplication of apparatus land programming of aproblem in a number of different ways, in order to check the operationof the machine.

As :a result of these requirements for accuracy storage mediums, suchlas magnetic tapes,-ar'e inspected thoroughly, and any defects thereinuusally mean that the tape cannot be used. If defects, or nodules, areunobserved, the data is incorrectly stored at that location and is aconstant source of error for the machine.

One method for checking tape which is known and used is to write a pulsepattern of a single binary digit over the entire tape. These pulses arelaid down .in the plurality of tracks or channels on the tape in whichnormally information -s to be recorded. Then the pattern 'of pulses isread out simultaneously by means of 1a magnetic transducer headpositioned over each channel. The outputs from these heads are appliedto a number of gates and to a counter, which function to actuate thecounter when all the heads do not simultaneously provide an output. Thusvall the imperfections on the tape which prevent a proper recording of apulse `are counted except that more than one vnodule simultaneouslypresent under the heads is countedl as one defect. This does not matter,since for the purpose of recording a defect in one or all of thechannels at a given location, since it is customary `to record binaryinformationr in parallel, renders that location useless. 'I'he tape isthen either -accepted 'cr rejected, based upon the nodule count and the.amount of error tolerancepermitted. If accepted with a maximum numberof nodules errors are bound -to occur, but this isV cheaper thanthrowing the tapes away until a perfect one is found.

- vAn object of the present invention is to `provide, la system foridentifying nodules in a magnetic recording medium so that errors duethereto may be avoided.

. Another object of the present invention is to provide apparatusfori-dentifying defects l in a magnetic recordingy medium, so that lessperfect recording mediums may be employed and still provide error-freeoperation.

Still another object of the present 'invention is to provide a systemfor checking defects in magnetic tape which is novel and useful.

Another source of trouble in magnetic tape is known as skew.V Thepresent system for utilizing tape is to use a plurality of tracks andsimultaneously record pulses, representative of b-inary digits, in thesetracks by means of magnetic heads which are aligned transversely acrossthe tape and with each head positioned over each track. The tape issubject to a number of stresses, both from rapid starting and stoppingand even in reading. Accordingly, when the tape is again passing underthe magnetic heads, one section or side of the tape may be stretchedmore than the. other. This causes the pulses in the tracks tobemisalgned to a certain extent. Accordingly, instead of pulses beingsimultaneously read from the tape exactly as they were recorded on thetape, there is a delay between the reading of the digit in each of thetracks which is proportional to the stretch or skew which the tape has.Another skew difficulty results from the tape passing the aligned headsat an angle which is different in recording than in playback. This eiectis a function of the tape transport mechanical design.

An object of the present invention is to provide a system forcompensating for skew.

Another object of the present invention is to provide a system fordetermining the amount of skew assumed by .a given tape.

The above and further objects of the invention ar'e achieved in a systemwherein the outputs of the magnetic reading heads zare applied throughan or gate to a l'rst variable time delay circuit to the trigger a firstpulse generator. The output of the first pulse .generator is employedlas va data sampling pulse. 'I'he magnetic head outputs are also appliedthrough an and gate to a second and gate .as an inhibiting input to thatsecond and gate. The pulse generator output is also applied to thesecond and gate. The output of the second .and gate is applied .to acounter and also to a recording head. Accordingly, if tape has beenprepared by recording `a pattern of pulses in every track, in readingthe tape this invention will count al1 the defects on the tape and willrecord a pulse or train of pulses, which locate this defect. With suchdefects being marked in this manner yalong a nodule marking channel, itis a simple matter to read the pulses on the nodule marking channel andemploy them to prevent recording on the tape location where the nodulesvare marked. In this manner, errors in reading from tape due to nodulesare eliminated. Also, even though a tape has va large number ofdefects,`it can still be employed and provide substantially error-freeoperation.

The novel features that are considered characteristic of this inventionare set forth with particularity in the appended claims. The invention,itself, both as to its organization and method of operation, as well asadditional objects zand advantages thereof, will be'best understood fromthe following description when read in connection with the accompanyingdrawings, in which:

Figure 1 is a schematic diagram of an embodiment of the invention, and

Figure 2 is a diagram of the Wave shapes which are obtained at variousportions of the circuit shown in Figure ll and which :are shown herein,in order to provide a better understanding of the operation `of thisinvention.

kReference is now made `to Figure 1, where for illustration of theinvention it is shown being employed with a..section of magnetic tape 10having `a plurality'of recording channels. As an illustrativeembodiment, four channels are considered, three of which may beconsidered data recordingchannels, and the fourth the nodule markingIchannel. The tape passes underneath the magnetic .transducer ,headsrepresented ...by rectangles. The lthree heads 112,14, ;16 for the datachannels .aresubstantially aligned .and .thehead .18 .for the nodule.channelis displacedslig'htly .from .the .otherheads in the directionfofmotion of the tape 10. The alignmentof `theheads can b e transverse, asshown,.or they may bepositionedin a skewfashionif desired. .They must bepositioned, however, .so that'a recorded digit in..a binary numberpasses undereach one of. themsubstantially simultaneously. The tape isprepared .for testing by writing pulses in the data recording.channelsa1ong .the `.entire length of the tape. yl'hese 4pulses may alllbe .representative of vbinary ones or binary zeros. y,'1`heirnp0rtantthing is that the same plscibe written bymeans of the substantiallytransversely aligned data headsin the data tracks. The writing may be.donebyeither using ythesame heads used for reading and writing or'byusingseparate heads for each. The tape is .thenrunpast thefour headsshownin Figure .1. The outputstrom the .threedataheads 12, 14, .16 areapplied vto.,thr.ee high-,gain.ampliiiers .22, 24, 26, each-of whicharnplifies the signal picked..up and applies it to a.separatelyg'associatednetwork known as .a.slicer.3,2, 34, .-36.

vv'The sliceris acircuitfor .permitting only that portion l'o`f.a signalto passthrough which exceeds .acertain minimum level and which does notexceed :a maximum level. A suitablecircuitof this type is described andshown on pages 45 and 46 of the book Waveforms byChancefet al.,,publishedby McGrawfHillBook Company in 1949. The .output.of.each.slicer is .applied to a separate ip-llopcircuit 42, 44, 46.These flipop circuits are of the `well knowntwo-tube Eccles-Jordan typeand may be .found described onpages 595 c0597 of ,the bookRadioEngineering by Terman andpublishedby McGraw-Hill Book lCompany in 1947.

`Each trigger circuit hasone input terminal designated lby "S vas the,setterrninaL to which the application of positive, pulses causes theflip-floptohave one of its tubes lconducting andthe other tubenonconducting. .Output 14is-.taken from the .anodeof the conducting tubeandap- .plied to afseparatecathode followerstage 52, 54,56. The other,tube gridofthetrigger circuitmay beconsidered asa resetgriddesignatedv.as R", input terminal,since application ,of apositivepulse .to the.other tube grid resets -the flip-flop to itsinitial .condition fromwhich it was ,.transferredby application of a pulse to its S input..TheinitiaLcondition maybe called a nonindicating con- -dition and .thecondition to 'Whichthe ipop is driven .by thev applicationof a pulse tovits. set terminal-may be Ydesignatedas the indicating condition.Accordingly, vwhen ,.the.`jiip.-flop in each channel isdriven to thevindicating conditionby an .output .from its associated slicer, an out-.put .is .appliedto -the .associated cathode follower. Each.cathodemfollowerSL .54, .56.. applies its .output to (1) an or gate 58;(2) a first and gate 60; and (3) .adata .reading .fand. .gate 62, ,64,66.

"Or. gatesand and gates are well known in the art, .and .asuitabletype-.for each maybe foundv described in an ,articlesentitled .Diodecoincidence and mixing circuits..for vdigital computers by .TungChangChen in the Proceedingsnf thelRE, May 1950, pagesSll to 514.

An .an Igate .is.a.coincidence.device lwhich provides '-anoutputonlywhen all.its;inputs are. present simultaneously. An or gate is a deviceor circuit whichprovides .an output whenanyone of its inputsparepresent. Thus, .the .for.`gate.58 providesanoutputwhen it,.receivesian.inputirom oneeof .thechannels from l the magnetic transducerheads`outputis applied to .an adjustable time .delay circuit v68. ,.An.adjustable time delay.circuit .Ofiatsitable type may be found describedin`.Electronic "Instruments .by Greenwood et al., published by 'Mc-GrawfHilliook Companyand mas" beJfOlmd Onfpages 591 et seq.

'4 This circuit is merely -a one-shot multivibrator which provides anoutput pulse at a time later than the application of an input pulse byan interval which can be determined or controlled by adjusting thecircuit component values.

The output of the adjustable time delay circuit 68 is applied tola'pulse `generator70. This pulse A)generator ,is wellknown.intheartvandmaybe ablocking oscillator of lthe type ,described ein.chaper .6 vof the .above noted book Waveforms. The output of the pulsegenerator is applied as a second input to the data and gates 62, I64,661and v`also to'asecondand gate-72. The 'second and gate`-hasasitsother. input lthe output of the rst and gate 60,which'has'been inverted through a phase inverter 74 andfthenijappliedthrough Ja ,cathode follower stage 76. Thisinverted input serves toinhibit the opera- 'tion offthel-second' andgate '72,'lwhich otherwisewill pass" the o utput ,of thejfirst ,pulse generator. This type. .ofand gate is`known as a but-not gate, `inthat it `will pass one of itsinputs but not both. This type of and L gate isfoundedescribed*,byfFelker invan articlein Electrical .Engineering for.December .1952, entitled Typical block. diagram .for .fai transistordigital computer.

The rst pulse generator output is also applied toa .second time-delay.circuit l78f vvhich issubstantially-similar to the first. Theoutputofsthe'secondttime delay. circuit `78is applied toa-.second,.pulse generator 8.0 .which also .issimilartothe.'rshbl'ocking oscillator. Its outputis vutilizedto.resetthefiipopcircuits 42, 44, :46 ,in v.each channel. .Its output, hereinreferredtto as .a data-shift ,pulse.has..other usesas well. .If,the-,apparatus shown is .used .solely .for ...the ,.purpose of readingthe tape, :the: iirst and second andgates V60,72'. and theapparatusfollowing them yis not -requireddata :is fread. as the outputs.of each `one .of .the data and.gates 62,64, 66. The data shift pulse.'rnay'fbe used* to transfer. the yinformation read4 from thefdata.andgates Yintoy subsequent apparatus, .to clear it .to ,receive inewdata. yAlso, .-both the data .sampling gpulse and the .data.shift=pulse.may be employed as clock pulses having a given desired intervalbetween-.them as determined-by the time delay .networks. These .clock.pulsesgmay be employed. in whatever. apparatus in which v theparticularsystem vshown ...and -described .herein .are

incorporated.

The rstwandf second and .gates operate to provide pulses formarking.anodulevchannel 'on the tape. The .first and gateyonlyprovidesfan outputfwhenallthe pulsesare present and readirornthedata channels. The `second and;. gate 72 provides-aan output pulse.anytime there-isa pulseinone ofthedatachannels being. read. In l'view.of .the'roperationof the rst time delay network 68, the'fdata-'samplingpulse does not loccur Vvuntil. the. data pulse :has existed for adesired interval. Accordingly, the'lsecond "andggate'can be .inhibitedbytheflrst .and :gate-.andnwill net epass the 'subsequently occurring.data lsampling-pulse if --uallfthe pulses occur. in-.the data track.:If .one .oft themdoes notjoceun due -to .thepresenceof ra 'nodule-.lorother ',impverfection, `the datasampling .pulse .passes through 'thesecond ?-and"gate .and then isapplied :by-means of fa; cathode' fo'llower182. to a counter'= 84 to -be counted. @Thessecond vandff'gateoutputgis .also applied through another time nielaynetwork.86-.similar.to the precedingffones .and fanother `cathode ifollower 88 tto the lnodule'-ma'x'lring magnetic yrecording yhead .18..The-mag- Lnetie recording *head records apulse int thenodule :chan--1-n`el. `The"reason *foreits displacementis `that thetape has travelleda"--small distance fin thefinterim required for `the marking apparatus-f-to *fbe Voperated. ""Thus the nodule` marking pulse., is appliedadjacent toftheI Alocation of' the'V nodule.A The number dfnodules arealso counted by the jcounter. 'Thus theI endproduct is fa tape which hasall its'nodule locations identified and counted. Of course, if desired,Y"othermeans' of -nodulef marking Vmay 'be'emp1oyed,==such as-an-actu`al marking, so =-.that

5. visual identification may be made. The tape may then be spliced, withthe imperfect portion cut out. Otherwise, with a magnetic nodulemarking, itis well within the skill of one versedin the art to requirethat the nodule channel be read in advance of writing on the tape. Theoutput from the nodule channel can then be used to prevent writing untilthe imperfect portion'of the tape has passedby. Another and also apreferred embodiment of the invention is to place the nodule markinghead slightly in advance of the nodule along the direction of motionlpulses have been recorded in the three channels, but the` third channelhas a nodule, and, therefore, no pulse is recorded transversely with thetwo pulses in the othertwo data channels. The magnetic flux in thetransducer head during playback is shown as the second curve 13 ofFigure 2. The output voltage .from the transducer head will have thewaveform 15 shown in the third curve. The output of the slicer will bethe rectangular pulses 17 shown in the fourth curve, and the-ilip-opwill provide the pulses 19 as shown in` thesixth curve. The datasampling pulse shown b y the seventh curve 21 occurs during the intervalof,the existence of the outputs from the flip-Hops. Accordingly, itfisduring -this interval that the data.and \gateswill. provide an output.The first and gatewill not provide l'an' output in view of the factthat.

all of its inputs are notfpr'esen't.i-"Accordingly, the second "and gatewill pass a pulse from the firs-t pulse generator which serves toprovide av marking pulse transversely aligned on the tape with" thedefective portion of the data channels. The reset pulse shown by thelast curve 23 occurs at a short time after the data sampling pulse andserves to reset the flip-Hops so they may be in condition for indicatingthe next signals coming under the magnetic transducer heads.

If the signals read by the magnetic transducer heads do not occursimultaneously as a result either of the skew lof the tape or of themisalignment of the recording heads (not shown), then the first an gatewill not provide an output and the indication is of the same type(noninhibiting) as if a nodule had occurred. By increasing lthe delay ofthe rst variable time delay network, the data s-ampling pulse can befurther delayed until Ian output is provided from the first and gate.Thus there is available a means of compensating for tape skew. There isalso available a means for determining the extent of the tape skew sincethe amount of delay required in order to permit the first and gate tooperate and inhibit the second and gate can be readily measured. Whenthis is considered together with the rate of tape travel, the amount oftape skew can be easily computed.

Although the system is here shown and described as one wherein thelocation of the nodules are identified by a pulse in the nodule channel,this also can be performed by marking a pulse everywhere along thenodule channel except where a nodule occurs in the data channels. Thusthe presence of the nodule is identified as the absence of a pulse. Thiscan be accomplished by omitting the inverter in Figure l and changingthe second and gate to be the same type as the first and gate.

It will be appreciated that the present invention is employed whererecording of closely spaced pulses on tape is desired, but not soclosely spaced as to smear together so that the pulses cannot be treatedas discrete units. Furthermore, if the test recording is made with largegaps between test pulses, the large gaps will remain untested and thesystem will provide' accurate results only if the laying down of pulsesis subsequently performed in the same regions as the ones in which theoriginal test pulses were laid down. The usual defects typically eectpulses over about 25 to 50 miles of tape and With the present recordingsystems between ve and ten pulses are usually packed in that space.

The lower limit settingof the slicer 32 can be employed, to a certainextent, to increase or decrease the number of nodules or tapeimperfections marked by the subject invention. The nodules on a tape arecharacterized not only by losing percent of the signal recordedthereover but also by causing a considerable reduction in amplitude ofthe recorded signal. Therefore, a determination should be made of theminimum amplitude signal to be tolerated on readout and the slicinglevel should be adjusted accordingly.

Accordingly, 'there has been described and shown above a system foridentifying, counting, and marking on tape the location of nodules sothat tapes which have heretofore been deemed not usable for accuraterecording and reproducingare now made usable. Also, compensation fortape skew and the measurement thereof is now afforded by means of theabove described system. The defect marking system herein obviously mayalso be employed with magnetic drums and other magnetic recordingmediums wherein recording is made in the manner,

described above.

I claim:

1. Apparatus for testing a magnetic recording medium employed in asystem wherein said recording medium has a'plurality of parallel datatracks, and a separate magnetic transducer head is positioned over eachof said tracks, said apparatus comprising means to record substantiallytransversely aligned pulses in all said data tracks, a means for eachchannel to generate a signal indicative of one of said aligned pulsesbeing read by the transducer for that channel, means to generate a datasampling pulse after a desired interval responsive to output from. anyyone of said means to generate a signal, means responsive to a coincidentoutput from all said means to generate a signal for each channel togenerate a coincidence indicating pulse, and means responsive to a datasampling pulse and to the absence of a coincidence indicating pulse tomark said magnetic recording medium at the location of the pulses whichfail to generate a coincidence indicating pulse.

l 2. Apparatus as recited in claim l wherein said means to generate adata sampling pulse after a desired interval includes an adjustable timedelay network tol permit determination of the extent of skew 0f saidmagnetic record ing medium.

3. Apparatus for testing a magnetic recording medium employed in asystem wherein said recording medium has a plurality of parallel datatracks, and a separate magnetic transducer head is positioned over eachof said tracks, said apparatus comprising, means to record substantiallytransversely aligned pulses in all said data tracks, a fliplop circuitfor each channel having a non-indicating and an indicating state, meansto couple each head to an associated flip-flop to drive it to anindicating state upon reading one of said pulses, means to generate adata sampling pulse after a desired interval responsive to any one ofsaid flip-flops being driven to `an indicating state, means responsiveto a coincidence of all said flip-flop circuits being in said indicatingstate to generate a coincidence indicating pulse, means responsive to adata sampling pulse and the absence of a coincidence indicating pulse tomark said recording medium at the location of the pulses which fail toprovide a coincidence indicating pulse, and means responsive to a datasampling pulse to reset said flip-flops after a predetermined intervalto their nonindicatng state.

4. Apparatus as recited in claim 3 wherein said means to generate a datasampling pulse after a desired interval emploi/'edi ufa system'"hereinfsidireeording medium; hasond-'time Ydelay' network;measftvceuple jsaidflfrst-andf gate output to said second and gate,means t'c'rni'arkA 7. Apparatusgfordetelingsand# indicating the presencedetermined llocations 'inf each*ofifsa'idarays,l said arrays' beingdisposed longitudinally along said medium,v said medium including anindex-'tradk'disposdlongitudinally endl, umefdely network? a said secondpulse generator toTgit'wte"apulse'responsive'1 thereto, and'm'eansf-'t'olapply' sid-iscond'pulse generator' for detecting a1:- d'efect"inll'anyi otteo: said; predetermined loctitiorts` of said one array. andfor"1 producing a'- firstV outpub signalA whenever a' defectisdet'ectedl thereby,v means'VA resp'onsivefftoany oneofIsaid-bits'of'saidmne array for providnga second-output signal;Aand-^m`eans` responsivel tothe simultaneous occurrence'of said first andsecond' output signalv orproducing` a third outputy signal to controlthe recording' of an index marker atv a`4 position along said `indextrack which lcorresponds to the longitudinali position of said-onearray;

8. Apparatus "for producing:asubstantially simultaneous deliverywof aplurality of signals-corresponding respectively tolthe' binary digitsstored i representative form ina transverse recording position onafrecording medium provided witha-plurality of substantially parallelinformation'storagetracks, said mediumfbeingemployed in'a system whereineach ofanv array of transducer heads is positionedoppositevr a=respeclive'-on'efof saidtracks Vand wherein relative`motion-*isfprovidedlfbetween'said heads andfsaid medium nadireetionsubstantially parallel to said tracks,` comprising-af plurality ofbistableI devices each' having twof stable states and'operable 'upon'receipt` of the output signals from a respective one of saidVtransducer heads` to assume thev stable state.*correspondingA to thestored binary digit? passingtthe-co'rresponding head lat that instanhardelay'signalgenerator v'coupledfto all of said bistable dvice's'and'responsive to the first of said bistable devices`- to 'assumef a'-predetermined stable state when ra` transverse recording" positionV onsaidu recording mediuml re'aohessad varray off heads 4for"generating"y asampling signalifafter a^predetermned delay, andVr means coupled10J-said delayfsignalgerteratnr andtoeach of said bistable devicesand'responsivet'.sidsatplingl signall for deliver`v ing-:substantiallysimultaneously'apluality of signals each` representing .the-statefof'afrespective'one of said Ibistable devices whenfsaidf samplingfsignal'occurs.U

9.' Apparatus as"'in"claimf 8 further including. means coupledto all ofsaid'bistable devices and responsive` to said sampling vsignal for Yresetting all --ofisad' f bistable' d'e` vices to acommo'npredeteminedstable stateafter said sampling-signal is' generated."

References vCited-l in the` `ile of vthis patent or the voriginal patentUNITED :STATI- ".Sy PATENTSy 2,528,2950'1' WiegandH Oct'. 3'1`, 19502,528,682 Bianey` Nov.` 7', 1950 2,628,346 Buridiart Feb. 10, 153.532,817,829" Lubki'n" Dec. 24, 1957

